Figure 1 | Hilltop homes in the province of Potenza (Image produced by author)

Italy is known for its rich architectural history. Today, many ancient and historical structures still stand. While the mention of Italian architecture often invokes thoughts of the more famous and well known structures, less consideration is given to the large quantity of buildings constructed in more recent centuries that are nonetheless historical. Structures throughout Italy boast a wide range of masonry styles and materials dating all the way back to medieval times. Hilltop towns in Southern Italy, like those in the province of Potenza for example, have a handful of historical brick and stone masonry homes and structures, as seen in Figure 1. As the 1857 Basilicata Earthquake has shown, however, not only are these buildings experiencing general deterioration, they are also susceptible to collapse under lateral seismic loads. In other cases such as the Palazzo Edilizia in Salerno, masonry deterioration can cause susceptibility to collapse due to inadequate structural strength. With many of these historical homes and buildings still in use, measures to ensure structural integrity and inhabitant safety are essential. These measures come in the form of masonry assessment, repair and strengthening.

Italian Masonry Construction Types

One of the first steps in assessing the masonry construction is understanding the materials used. Identifying the type of masonry units used, where they came from, what the material properties are, and general performance strengths and weaknesses of the material all help to achieve this understanding. Local rehabilitation experts and others in the construction field are good resources to have in determining the type of masonry units used. Stone identification charts, as well, may be used to supplement or reinforce local knowledge (London 1988, p 43).

Stone Masonry

Since ancient times, stone masonry has been used with varying popularity. Throughout Italy, a wide variety of masonry buildings from multiple historical eras exist today. Italian stone masonry may be composed of rubble stone with mud/lime mortar, or two-wythed exterior walls that use rubble infill in mud/lime mortar, but the type of stone used and building styles vary depending on the region (Lutman 2011, p 2). Limestones and sandstones are a popular choice in Tuscany, for example. In Val Trebbia, early medieval masonry structures consist of two wythes of marly limestone ashlars built directly atop the ground, with mortar placed in the wall cavity and a timber structure (Cinieri 2014).

Stone masonry walls will typically have no support or be supported by stone masonry strip footings. Timber joists placed directly on structural walls typically support upper stories and often are retrofitted with reinforced concrete slabs or precast concrete joists (Lutman 2011, p 3).

Figure 2 | Brick masonry home in southern Italy (Source by author)

The performance of the stone masonry, as with any masonry, is heavily related to the type of stone used and the mortar it is paired with. Igneous rocks, such as granite, tend to be more homogeneous and hard. Sedimentary rocks, such as limestone, are composed of non-homogeneous layers of varying material. Metamorphic rocks are rocks that have experienced great heat and pressure thereby transforming them into harder and denser rocks, such as marble (London 1988, pp 45-46).

Unreinforced Brick Masonry

Brick masonry, as well, is a popular building material. Depending on the area, brick sizes used and the types of clay the brick is made of can vary, as can the mortar composition. Mortar experiences the greatest variation, as the mix ratio of sand, water, and the bonding agents used often depends on local material availability and practice. Brick masonry walls in historical buildings may consist of one or two layers of brick or of two wythes with a rubble-filled cavity for thermal performance (D’Ayala 2011, pp 1-2).

Analysis

Analysis to Determine Appropriate Solutions

In order to ensure lasting, durable repairs, a solution that addresses the cause of deterioration is necessary. Spot patching of masonry, for example, is not an effective or long-term means of repair. In Italy's historic buildings, general deterioration that comes with age is likely to be seen. This may involve deteriorated mortar for example, which often calls for repointing between the brick or stone units. Not all appropriate repairs will be this straight forward to determine, however, and care must be taken to ensure the correct issue is being addressed (London 1988, pp 63-65).

The first step in diagnosing the issue is understanding the construction of the building. Masonry construction methods vary depending not only on where the structure was built, but on when in Italian history it was constructed as well. If the history of the building is known, the ability to choose compatible repair materials is significantly enhanced (London 1988, p 67).

Analysis of Stone

Understanding the stones behavior is an important part of discovering the source of deterioration. Key properties are listed below in Table 1.

As the table describes, in the case of bedding planes, breakage parallel to the planes may occur. This may be seen in Figures 3 and 4, where a long crack runs through the center of the stone unit. Identifying the failure type being expressed helps to determine the cause of failure. In this case, discoloration - another characteristic of deterioration described in table 1 - can also be seen along the crack. This could signify moisture presence, and further analysis would need to be done to find whether the moisture was a cause or a result of the cracking.

Common Sources of Deterioration

Masonry may deteriorate due to a wide variety of reasons. These may include moisture retention that creates efflorescence or results in freeze thaw, differential settlement or deterioration of other materials in the wall due to water flow, thermal expansion, deterioration due to wind, deterioration due to exposure to pollutants in the air, and general wear and tear (London 1988, pp 66-67).

Means of Analysis

A Variety of methods exist to evaluate the performance of the existing masonry. These methods can be either destructive or nondestructive in nature. Often removal and testing of the existing masonry is undesirable, which is why nondestructive techniques may come into play. However, both destructive and nondestructive techniques aim to evaluate the composition and structural bearing capacity of a wall. Various means exist which estimate the metal embedment locations and sizes, the occurrence of internal irregularities, wall thickness, the extent of cracks or voids, and moisture content (Schuller 2012, p 1).

Nondestructive Analysis Methods

Nondestructive test methods are particularly useful for historical structures that do not allow for removal of wall parts for testing. In these situations, in-place test methods, such as those listed below, may be used. As the table describes, surface penetrating radar uses the reflection of microwaves in order to determine embedment location and depths. As figure 5 demonstrates, this can then be used to predict the layout of the wall and location of elements within it. Another means of nondestructive testing listed in table 2 includes measurement of rebound hardness. Figure 6 demonstrates the use of a pendulum hammer in measuring hardness.

Repair

Due to the wide variety of masonry materials and mortars used, special care must be taken to assess and develop a solution appropriate to the building under consideration, otherwise additional damage may be introduced. Adding mortar injections without consideration for the original mortar’s composition, for example could cause additional stress in the brick, resulting in cracking and additional deterioration. Proper masonry repair involves great care in ensuring material compatibility. Once compatibility is determined, strengthening and repair may be done in various ways, including the addition of ties between wythes of walls, filling of voids, cement grout injection, and the addition of steel reinforcement if necessary (Schuller 2012, pp 2-.3).

Stone and Brick Masonry Repointing

Once the cause of deterioration is addressed, deteriorated mortar joints will call for repointing, or replacement of the mortar with new mortar. In order to execute this process, assurance of mortar compatibility with existing materials is critical, otherwise more damage may be incurred by the structure.

The first consideration in the choice of mortar is strength. Mortar joints must be weaker than the masonry units themselves, otherwise the mortar does not obtain enough moisture permeability, and unit failures may occur from moisture concentrations developing within them (London 1988, p 11). The next step in choosing a mortar is compatibility. Historic Italian structures often utilize a mud/lime mortar mix or no mortar at all (Lutman 2011, p 2). If the original mortar mix can be determined, that is often a good way to determine the recipe. The most essential part in choosing a new mortar is ensuring it is weaker than the masonry units and that it is at least as soft as the existing mortar (London 1988, p121). Additives such as those that increase curing speed or modify color are not recommended for historic structures (London 1988, p 115).

Rehabilitation to Increase Seismic Performance

With a large quantity of earthquake-prone regions in Italy, measures to strengthen masonry structures while preserving their historic character is a must. Unreinforced masonry performance under seismic loads relies on how well the wall is tied together and anchored to the roof and floors. Poorly anchored and tied structures are the cause of multiple failure types, including failure at wall intersections. The large amounts of stress that occur at wall intersections leave these areas particularly susceptible to failure under seismic loading. This may result in large vertical cracks or separation at walls that are not tied together well. Use of different multiple masonry types at intersections - a practice commonly seen in Italy - may even result in worse seismic performance. Other failure types that may result include out-of-plane collapse and roof collapse (Bothara 2011, pp 16-19).

Diaphragm Strength

Along with the lack of significant tensile strength in the masonry walls, the use of flexible wooden floors contributes to poor performance under seismic loading. One step in mitigating this is increasing the diaphragm strength, thereby decreasing horizontal displacement and reducing the risk of wall collapse due to excessive deformation. This can be done in multiple ways, but composite slab on steel beams and concrete slabs will typically exhibit behavior closest to a rigid diaphragm and give the least horizontal displacements (Branco 2011, pp 1628-1630). The methods that are commonly practiced in Italy, however, involve retrofit with a precast concrete joist system or reinforced concrete slabs (Bothara 2011, p 6).

Strenghtening of Masonry Walls

Strengthening of the masonry itself may also be done with use of base isolation, viscous dampers, or concrete core walls anchored to the masonry walls in addition to or in lieu of strengthening the floor diaphragm, as shown in Figure 5. Though the use of viscous dampers does not provide as significant improvement in seismic performance as concrete walls or base isolation, it does have the benefit of minimal impact on the existing structure by providing ease of installation and little disturbance of the existing structure (Branco 2011, pp 1630-1631).

Although the techniques mentioned in the section above create more drastic improvements in seismic performance, they are also more intrusive aesthetically. Methods using polymer grid reinforcement not only have seismic benefits, but they improve the overall durability of the masonry while maintaining the original aesthetic as well (Juhásová 2008, p 1).

Case Studies

Casa Capello

Casa Capello, located near Turin, is a 14th century building that was retrofitted at the end of the 18th century that has recently been restored and enlarged. Following the collapse of one of the structure's walls, cracking within the masonry had developed, and subsequently a need to determine the structural damage. Using the acoustic emissions technique, the amount of damage and extenuation of cracking was able to be assessed. This technique relies on sensors that measure the emission of elastic waves that are emitted right before damage occurs in a structure. The use of this technique provided a convenient way to monitor the evolution of cracks in the structure in a nondestructive way, while still allowing for structural evaluation (Carpinteri 2006, pp 1-2).

Palazzo Edilizia

In June 2007, Palazzo Edilizia - located in Salerno, Italy - experienced the partial collapse of six stories at one of it's corners. This collapse was precluded by large amounts of cracking throughout the structure that were identified on multiple occasions. The cause of the collapse was deemed to be due to a number of reasons, such as addition of large openings in the collapsed corner in 1955, few vertical diaphragms, and masonry piers at the ground floor that were smaller than those in the floors above (Palmisano 2014, pp 1-4).

Preservation in Italy Today

In the past, care has been taken only to preserve those buildings that are more iconic, but in more recent years there has been more emphasis on the value of preserving all historic buildings as part of the cultural heritage. As a result, there is greater emphasis on using the appropriate and most effective means of preservation and seismic rehabilitation, and the performance of these methods over time and after each subsequent earthquake is continually assessed (Anzani 2014, p 1).

Journal Article: In the past, measures had only been taken to preserve monumental buildings. Today, however, more emphasis is placed on restoring all historic buildings. This article provides short case study descriptions of such restoration.

Book: Research on the seismic retrofitting of historical structures. This book includes information on how to select the most appropriate seismic evaluation and rehabilitation with respect to the specific type and construction of a building.

Report: Overview of the types of stone masonry construction that can be found in Italy and other parts of Europe, as well as in Asia. This report also describes the seismic performance and strengthening of stone masonry structures.

Palmisano, Fabrizio, and Amedeo Vitone. "Partial Collapse of One of the Most Important Historical Buildings in Salerno, Italy." Journal of Performance of Constructed Facilities 28 (2014).

Journal Article: Investigation and analysis of the partial collapse of a late 1920's masonry palazzo in Salerno, Italy.